Mine doors operate under conditions not usually encountered by ordinary doors. A mine door leaf can be subjected to large forces due at least in part to air flow in the mine and consequent air pressure differentials on opposite sides of the door leaf. Mine door leaves can be as large as twenty feet wide and twenty feet high or even larger. Because of their large size, even small pressure differentials result in large forces acting on the door leaves. Mine door leaves have to be sufficiently robust in construction to withstand these large forces. This means that the door leaves tend to be fairly heavy. For example, a door leaf constructed for operation with a pressure differential of twenty inches water gauge may weigh up to two thousand pounds.
The weight of the door leaves in combination with the forces generated by air pressure differentials in the mine makes it difficult to control movement of the door leaves during opening and closing of the door. Likewise, it can be difficult to start the opening movement and complete closing movement of the door leaves. Thus, it is desirable for the opening and closing of the mine doors to be powered by one or more fluid-driven actuators. Hydraulic actuators offer some advantages over pneumatic actuators because the hydraulic fluid is substantially incompressible, making hydraulically-controlled mine doors less susceptible to door leaf runaway.
Mine doors are sometimes installed in relatively cold environments. For example, mines using forced air ventilation systems require doors to be positioned at openings from the surface into the mine to make sure that the air forced into the mine flows through the mine to the intended exhaust outlets rather than back out of the mine through an opening near the forced air inlet. Doors at the openings into the mine may be subjected to cold temperatures (e.g., as low as −50 degrees Fahrenheit) from time to time. Cold temperatures present a problem for operation of hydraulically powered mine doors because the hydraulic fluids used to operate the doors have substantially increased viscosities at these cold temperatures, making the hydraulic fluids too stiff to operate as desired. The lower limit of an acceptable temperature range for a hydraulic fluid varies depending on the characteristics of the particular fluid used in a hydraulic system. Fire-resistant hydraulic fluids, which are required for some mining environments, are particularly susceptible to this problem. However, non-fire resistant hydraulic fluids are also susceptible to cold temperatures.
One partial solution to the problem is to use a tank heater to heat the hydraulic fluid in the reservoir. Unfortunately, this solution does not adequately address all aspects of the problem because the fluid in the hydraulic fluid lines can also be cooled by exposure of the fluid lines to the cold. Further, some hydraulic mine door installations have long hydraulic fluid lines. For example, a single pump may be used to operate the hydraulic actuators for two (or more) different doors in an airlock, as described in more detail in U.S. Pat. No. 6,425,820, the contents of which are hereby incorporated by reference. Moreover, the doors in a mine airlock are often several hundred feet (or more) apart from one another to allow long trains and/or caravans of vehicles to pass through the air lock. Thus, long hydraulic fluid lines are needed to connect the pump to the various doors.
Because of the long fluid lines, a significant amount of hydraulic fluid is contained in the fluid lines (where it receives substantially no heating from the tank heater) rather than in the heated reservoir. Further, the difficulty of moving cold stiff hydraulic fluid through the fluid lines is exacerbated because the long lengths of the fluid lines are associated with a substantial resistance to flow that is independent of the increased viscosity of the hydraulic fluid therein.
Thus, there is a need for hydraulic door installations in general and hydraulic systems for operating doors that facilitate operation thereof in cold environments.